I had WFD  as a kid. My parents were told when I was six that I'd grow out of it and didn't pursue treatment. Now I'm an adult with a moderate-severe stutter.  From what I know, if you try and "treat" it, it can make it worse. If you don't "treat" it, that can make it worse too. Luck of the draw I'm afraid.  My advice is to focus on his confidence and mental health rather than getting fixated on his fluency.